The invention relates to a process for the thermal annealing of at least one implantation-doped silicon carbide semiconductor in a gas stream.
silicon carbide (SiC), preferably in monocrystalline form, is a semiconductor material with outstanding physical properties which make that semiconductor material of interest particularly for optoelectronics, high temperature electronics and power electronics. While silicon carbide light-emitting diodes are already commercially available, there are not yet any commercial silicon carbide-based power semiconductor components. That is primarily due to the elaborate and expensive production of suitable silicon carbide substrates (wafers) and the more difficult process technology in comparison with silicon.
One of the problems is presented by the doping of monocrystalline silicon carbides. Due to the high temperatures required, which are in excess of 1800xc2x0 C, it is practically impossible to dope silicon carbide by diffusion, unlike the case with silicon. Monocrystalline silicon carbide is therefore doped either by adding dopants during growth, in particular during sublimation growth (PVD) or chemical vapor deposition (CVD), or by implanting dopant ions (ion implantation).
The implantation of dopant ions in monocrystalline silicon carbide substrates or in a previously grown silicon carbide epitaxial layer allows targeted lateral variation of the dopant concentration, thereby making it possible to produce semiconductor components with a surface patterned in a planar manner. That constitutes a basic precondition for the fabrication of most semiconductor components. However, a problem with doping by implantation is the crystal defects (lattice defects, crystal imperfections) which are created in the silicon carbide crystal of the epitaxial layer by the dopant atoms implanted with high kinetic energy and which impair the electronic properties of the implanted semiconductor region and therefore of the whole component. Moreover, the dopant atoms or atomic residues are not incorporated optimally in the silicon carbide crystal lattice after implantation, and therefore only some of them are electrically activated.
Processes have therefore been developed for annealing the crystal defects created by the implantation by using heat treatment and, at the same time, for obtaining a high activation coefficient of the dopant atoms (so-called thermal annealing).
On one hand, an article in xe2x80x9cIEEE Electronic Device Lettersxe2x80x9d, Vol. 13, 1992, pages 639 to 641 discloses a process for the thermal annealing of a 6H-silicon carbide semiconductor region, which is n-doped by implantation of nitrogen ions at high implantation temperatures of between 5000xc2x0 C., and 1000xc2x0 C., in a 6H-silicon carbide epitaxial layer that is p-doped with aluminum. In that process, the 6H-silicon carbide semiconductor is treated at a constant annealing temperature of between 1100xc2x0 C., and 1500xc2x0 C., in an argon atmosphere. In order to prevent the surface from being destroyed by uncontrolled evaporation with the formation of craters and cavities, the 6H-silicon carbide semiconductor is introduced into a crucible made of silicon carbide. During the heat treatment, the surface of the 6H-silicon carbide semiconductor is in equilibrium with the silicon carbide atmosphere within the crucible.
On the other hand, an article in xe2x80x9cApplied Surface Sciencexe2x80x9d, Vol. 99, 1996, pages 27 to 33 describes the influence of the gas composition during the cooling operation of a chemical vapor deposition process (LPCVD=Low Pressure Chemical vapor Deposition) on silicon carbide semiconductors. The cooling operation starts at a maximum temperature of 1450xc2x0 C., which is thus comparable to the temperatures during thermal annealing after ion implantation. Therefore, the results that are obtained can also be transferred to the thermal annealing processes after ion implantation. In the investigations cited, it was ascertained that at temperatures of above 1000xc2x0 C., in vacuum or under a protective gas, the silicon carbide atomic layers near the surface are depleted of silicon, and a thin graphite layer can form on the surface of the silicon carbide semiconductor. If, on the other hand, the same process is carried out under a pure hydrogen atmosphere, then the result is a virtually stoichiometric surface.
It is accordingly an object of the invention to provide a process for the thermal annealing of implantation-doped silicon carbide semiconductors, which is improved in comparison with the heretofore-known processes of this general type and in which the formation or the clustering of undesirable crystallographically oriented steps is reduced.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for the thermal annealing of at least one implantation-doped silicon carbide semiconductor in a gas stream, which comprises holding the at least one silicon carbide semiconductor with a carrier within a container; conducting the gas stream within the container causing the gas stream to contact regions of the carrier and the container; forming the carrier and the container of a material selected from the group consisting of at least one metal and at least one metal compound, at least in the regions; and supplying practically no carbon to the at least one silicon carbide semiconductor through the gas stream.
The annealing process is thus to be configured in such a way that practically no carbon is supplied to the at least one silicon carbide semiconductor through the gas stream. In this connection, xe2x80x9cpractically no carbonxe2x80x9dis to be understood as a smaller proportion of carbon than that which corresponds to the equilibrium partial pressure of carbon or carbon-containing components (e.g. SiC2) over the silicon carbide semiconductor at the respective process temperature.
In this case, the invention is based on the insight that the crystallographically oriented steps which are always present in misoriented silicon carbide surfaces of layers applied epitaxially, for example, or of monocrystalline substrates and which ideally have a height of just one to two monolayers, cluster in an undesirable manner up to a step height of approximately 50 nm (step bunching) due to thermally activated surface redistribution. This takes place if, during the thermal annealing operation, the silicon carbide semiconductor is in equilibrium with a silicon carbide atmosphere or the gas stream supplied contains proportions of carbon which are at least comparable to this equilibrium state. Many small crystallographically oriented steps conglomerate in this case to form a few high crystallographically oriented steps. The small crystallographically oriented steps having a height of approximately two monolayers are an unavoidable consequence of the misorientation of the base silicon carbide crystals which is necessary for epitaxial layer growth. It has been found that the step growth described can be considerably restricted by reducing the proportion of foreign carbon in the gas stream, that is to say the proportion of carbon which is supplied externally to the silicon carbide semiconductor.
Consequently, when the gas stream is provided according to the invention, the step heights which result after thermal annealing are significantly smaller than in the prior art, in particular at least a factor of three smaller.
In accordance with another mode of the invention, at least the surface of the doped region of the silicon carbide semiconductor is exposed to a gas stream which preferably contains at least one inert gas and/or nitrogen and/or hydrogen. The gas stream composition can be changed during annealing, for example from an inert gas composition to a hydrogen-containing composition or even into practically pure hydrogen. Argon or helium with proportions by volume of up to approximately 100% are advantageously used as the inert gases.
In accordance with a further mode of the invention, a preferred variant of the process control resides in effecting heating in an inert gas stream, then maintaining an approximately constant maximum temperature, and subsequently effecting cooling in a gas stream with a proportion of hydrogen of typically at least 50%, in particular more than 80%, and preferably above 95%. Cooling in a hydrogen atmosphere results in a stoichiometrically virtually intact surface of the silicon carbide semiconductor, whereas cooling in e.g. an argon atmosphere may possibly lead to a thin graphite layer on the surface of the silicon carbide semiconductor due to silicon depletion.
In accordance with an added mode of the invention, in order to prevent dopant atoms from exiting from the silicon carbide semiconductor, atoms which have also been used for doping can be added to the gas stream under a predetermined gas partial pressure.
In accordance with an additional mode of the invention, the flow rate of the gas stream is preferably set between approximately 0.5 cm/s and approximately 60 cm/s, in particular between 5 cm/s and 25 cm/s. It has been shown that a silicon carbide semiconductor that is annealed under such a gas stream has a significantly better surface than a silicon carbide semiconductor that is annealed in a gas stream with a different flow rate. The advantage of having a gas stream flowing at the silicon carbide semiconductor during annealing is that, in contrast to the known annealing processes, in spite of the high temperatures, the surface has a good morphological quality, and the crystallographic steps stemming from the misorientation of the silicon carbide surface are essentially preserved and do not cluster to form larger steps, and other surface roughnesses are not produced either. The above-mentioned preferred range for the flow rate ensures that the flow rate on one hand is low enough to avoid impermissible cooling of the silicon carbide semiconductor, and on the other hand is high enough to transport away carbon and silicon atoms exiting from the silicon carbide semiconductor, so that they cannot contribute to undesirable step growth.
In accordance with yet another mode of the invention, the static process pressure in a region of the gas atmosphere adjoining at least the silicon carbide semiconductor is generally advantageously set between approximately 5000 Pa and approximately 100,000 Pa (normal pressure) and preferably between approximately 10,000 Pa and approximately 50,000 Pa. The negative pressure which is set ensures that the undesirable growth of the crystallographically oriented steps is suppressed particularly well.
In accordance with yet a further mode of the invention, the silicon carbide semiconductor is disposed in the interior of a container which can preferably be heated through the use of an HF (High frequency) induction coil. The silicon carbide semiconductor is preferably held by a carrier in the interior of the container.
In accordance with yet an added mode of the invention, at least one radiation shield is placed in the interior of the container, in each case upstream and downstream of the carrier, with reference to the direction of the gas stream, in order to prevent an undesirable radiation of heat from the interior of the container. openings for the passage of the gas stream are preferably provided in the radiation shields.
In accordance with yet an additional mode of the invention, the carrier, the radiation shields and the container, for example at least parts of the inner wall surface of the container, are advantageously composed of at least one metal or at least one metal compound, or are at least lined or covered with the same, at least in the regions which come into contact with the gas stream.
In accordance with again another mode of the invention, the metal or the metal compound should advantageously melt only at a temperature in excess of 1800xc2x0 C., due to the high process temperatures during thermal annealing.
In accordance with again a further mode of the invention, the metal or the metal compound should advantageously have a vapor pressure of less than 10xe2x88x922 Pa (approximately 10xe2x88x927 Atm) at the maximum temperature of 1800xc2x0 C.
In accordance with again an added mode of the invention, the metal or the metal compound should advantageously be resistant to hydrogen due to the proportions of hydrogen that are provided in the gas stream.
In accordance with again an additional mode of the invention, metals or metal compounds which contain at least one of the materials tantalum, tungsten, molybdenum, niobium, rhenium, osmium, iridium or carbides thereof can thus be used with particular advantage. Parts of the container which do not come into contact with that part of the gas stream which reaches the silicon carbide semiconductor may also be composed of different materials such as, for example, graphite or silicon carbide. All parts which have not been mentioned before but may be present in the hot region and come into contact with the gas stream should likewise preferably be composed of the above-mentioned advantageous metals or metal compounds or at least be coated with the same. The advantageous material selection that has been described ensures that the gas stream which flows past detaches no carbon atoms from the contact areas, such as the inner wall surface of the container or the carrier surface, or takes up carbon atoms that have emerged and takes them to the silicon carbide semiconductor.
In accordance with still another mode of the invention, an implantation-doped silicon carbide semiconductor is brought to a maximum temperature of at least 1000xc2x0 C. by supplying heat. The increase in temperature with respect to time (heating rate) is generally restricted to at most 100xc2x0 C./min, preferably to at most 30xc2x0 C./min, during this heating process.
In accordance with still a further mode of the invention, the maximum temperature is advantageously set between 1100xc2x0 C. and 1800xc2x0 C., preferably between 1400xc2x0 C. and 1750xc2x0 C.
In accordance with still an added mode of the invention, the silicon carbide semiconductor is advantageously kept at least approximately at the maximum temperature for a predetermined time interval of preferably between 2 min and 60 min, in particular between 15 min and 30 min. This high temperature plateau provides an improvement in the activation coefficient of dopants in the silicon carbide semiconductor.
In accordance with still an additional mode of the invention, the cooling rate is advantageously limited to at most 100xc2x0 C./min, in particular to at most 30xc2x0 C./min. The slow cooling operation expediently ends at an intermediate temperature of preferably below 600xc2x0 C. The restriction of the rates of temperature change (heating and cooling rates) leads to improved electrical properties of the silicon carbide semiconductor which is doped by implantation and then annealed.
The heating and/or cooling rate need not be constant, but rather may also advantageously vary within ranges defined by an upper limit of 100xc2x0 C./min and, in particular, by an upper limit of 30xc2x0 C./min.
In accordance with a concomitant mode of the invention, during the heating and cooling operations, the temperature of the silicon carbide semiconductor is kept in each case at a predetermined temperature level in each case at least once. The heating and cooling rate, respectively, is practically 0xc2x0 C./min during the period of time at which this temperature level is maintained.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for the thermal annealing of implantation-doped silicon carbide semiconductors, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.